I’M ON MY third generation of seed-starting lights, a journey that began back in the day when shop lights with so-called cool-white and warm-white fluorescent tube combined were all we gardeners knew. Eventually I moved up to newer, high-output fluorescents—and now maybe the switch to LEDs beckons.

But how does a person shopping for grow lights find his or her way through the array of possibilities out there? That’s today’s topic, with horticulturist Leslie Halleck, author of the book “Gardening Under Lights” (affiliate link) to help simplify things.

Sufficient light is maybe the biggest factor in the equation of success with seedlings and grow light technology is evolving fast. I asked Leslie to return to the show for a 101 on shopping for seed-starting lights suited to us as home gardeners.

“It’s all about accumulating a volume of light over a specific period of time on your seedlings,” she says—whichever lighting technology you decide to use. (Above, recently potted-up seedlings under lights at Leslie’s.)

Plus: Enter to win a copy of “Gardening Under Lights” by commenting at the bottom of the transcript.

Read along as you listen to the January 18, 2021 edition of my public-radio show and podcast using the player below. You can subscribe to all future editions on Apple Podcasts (iTunes) or Spotify or Stitcher (and browse my archive of podcasts here).

grow lights for seed-starting, with leslie halleck

Margaret Roach: Hi, Leslie, and boy, I hope you have the answers, because I’m confused [laughter].

Leslie Halleck: Hi, Margaret. Thanks for having me back. And I had to write a whole book on it because it’s just so confusing. Lighting, light science and grow lighting for plants can be a daunting topic for sure.

Margaret: Yes. Lots of science. When did the book come out? I can’t remember what year that was.

Leslie: 2018.

Margaret: Two-plus years.

Leslie: Yeah.

Margaret: Yeah. So when we talked about it, when it first came out, we did an overview of different applications of lighting and a little more of the, I guess it’s physics—is it physics of lighting?

Leslie:  Yes.

Margaret: And so this time I wanted to zero in on seed-starting and I don’t even know where to begin, but maybe… why might I want to trade up to LEDs, because I have these T5 HOs the high-output T5s and I love them. They’re fine. But even if I love them, every however many years, you need to replace your bulbs. And I was thinking, well, if it’s time to replace my bulbs, blah, blah, blah. So why might I want to change out my lights?

Leslie: When it comes to grow lighting and light available for your plants, it all comes down to efficiency. And when I use the term efficiency in relation to grow lights, I don’t mean necessarily how much energy they use. What I mean is how much energy they convert to light versus heat. That determines the efficiency. So the whole point of using a grow light is to get as much PAR—the light that drives photosynthesis in your seedlings [the acronym is for photosynthetic active radiation]—out of that lamp, versus heat.

The more efficient a grow lamp, is the more of the energy it pulls, it converts to light, usable light, instead of heat.

Many of the new quality-made LEDs are becoming much more efficient at delivering more light for the same energy usage, versus fluorescent, and obviously incandescents, which are very inefficient for photosynthesis. So there’s a lot of new developments in all types of grow lighting. Fluorescent included.

So it’s the HO T5 fluorescents that you’re currently using—and HO stands for high output. The shop lights you referred to are T12 and T8, and those are the bigger tubes. The bigger the fluorescent tube generally, the less efficient it is for our purposes. So it’s actually the smaller T5s that are the more efficient fluorescent lamp.

Now, those are still good options, and there are newer options as well on the market that are more efficient than older generations of T5 fluorescents. So there are still good options and it’s going to be the lowest-cost entry point for you as a seed-starter. So if you’re really looking for a low-cost option entry point, the HO T5 fluorescent, specific for growing plants in the cool spectrum, 6,400K range, are going to be a good option for you. O.K., but if you are already in a place where you need buy new lamps, because they do degrade over time.

They’re not going to keep putting out the same amount of light forever, right? Then switching to newer or more efficient LEDs is a good option. Now they’re generally going to cost you more; they’re generally going to be a little bit more expensive than fluorescents. We’re paying for innovation costs still with LEDs. But the goal is that you’re going to get more efficacy out of these lamps than you will with your fluorescent.

So efficiency in terms of how much light output you’re getting from that LED versus how much energy you’re putting in it, is improved over older fluorescent lamps. So the goal is that you’re going to hit a few birds with one stone, get more efficient usage for the energy that you’re putting into the LEDs, perhaps get a longer lifetime out of the lamp, and be able to perhaps use fewer lamps with the LEDs than you would the fluorescents, because they’re able to put out more light. Does that make sense?

Margaret: So, fewer tubes parallel to each other, hovering over our seed trays, so to speak.

Leslie: Yeah, we can jump right into that, too. So for example, a classic setup that I would use for HO T5 fluorescent lights for seed-starting is usually about four, 45-watt fluorescent tubes. That sound familiar?

Margaret: Yes, right.

Leslie:  … over a couple of square feet, right? So if you are using a 42-watt LED bar that has a higher-output capacity than that fluorescent, you might be able to use two or three of those LEDs versus four fluorescents. That is going to depend on the wattage. If it’s a higher-wattage LED it has the potential to put out more light. So if you’re using a higher-wattage LED bar, you might only need one or two. It all depends on the output capability of that grow lamp. And the more wattage it pulls, the more light output you’re typically going to get.

Margaret: So in my current hood—I have a hood, a good-quality hood [above], that reflective kind of hood that has four T5 high-output bulbs in it. And I have a-

Leslie: A fixture, a T5 fluorescent fixture. Yeah.

Margaret: Yes. And I understand now that there are some LEDs… I don’t know if you say tubes or bulbs or whatever—strips—that have the same little end hardware that could potentially, I could retrofit this thing. Would I use the same number, if it fits four of those? Or I’d only put in two, or is it an experiment [laughter]?

Leslie: Yeah. So, you can replace if you have a good-quality, T5 fluorescent fixture, the fixture that you pop your little T5 fluorescent bulbs, the lamps, into the fixture, they make what are called HO T5 LED retrofit bars. You have to be careful when you’re buying them, because standard LED bars have to typically be wired into a fixture or they’re plug-and-play—they come with their own plug and you plug them directly into the outlet.

If you want to continue to use your fluorescent fixture, your T5 fluorescent fixture, you need to look for LED retrofit bars. And you know the two little prongs that you see on the end of your fluorescent tubes that pop into that fixture? The LED retrofit bars will also have that same mount. And so you’ll pop that fluorescent tube out. You’ll pop the LED in.

Now I will warn you that sometimes there can be a little bit of incompatibility. A lot of the T5 fluorescents are 45-watt, if I’m not mistaken, and LEDs could be 42 watts. So sometimes I’ve had some trouble with older fixtures, but if you have a relatively, within the last few years, fluorescent fixture, you can try using the LED retrofit bars.

Now, chances are those LEDs are going to be somewhat higher output, O.K. Because they’re more efficient than your fluorescent so you might’ve had four T5 fluorescents in that hood [below]. You might only need to use two LED bars.

Margaret: Right. That’s what I was just thinking from what you said just a minute ago is that, “Oh, huh.” And that’s another place where I didn’t even know that was something I had to think about [laughter].

Leslie: Well, it’s all kind of balanced. So I think the way to reframe mentally, when thinking about light for seedlings or any plants, is that it’s not about brightness. It’s about volume of light, the intensity of light that’s going to fall onto the leaves of that plant. The more efficient a grow lamp, the more photosynthetically active radiation, the more light that that plant uses to grow, it can put out.

So if that grow lamp is more efficient and puts out a higher volume of light, then you can use fewer lamps, or run it for a shorter timeframe. If you have a lower-wattage, lower-output lamp, you may have to put it closer to that plant and you would have to run it longer. It’s all about accumulating a volume of light over a specific period of time on your seedlings.

Margaret: O.K. So you mentioned one of the downsides of the oldest fluorescents, and even some of the newer ones like I have currently, they emit more heat than the LEDs, the good-quality LEDs do.

Leslie: Right.

Margaret: And so the baby plants don’t necessarily love that. And so it will be nice to have less of the heat near my seedlings, not having to put the lights so close to them, either. But there are other advantages. For instance—and I was surprised at this, actually—that how many of my readers…. I do this column in “The New York Times,” and I had just mentioned in passing about seed lights that I used the T5s. And people in the comments just like jumped right in and said, “How can you use those? There’s mercury in the tubes. They’re so bad.” And so there’s other reasons why LED technology may be preferred, right? Environmentally speaking, not just the usage of electricity?

Leslie: Well, I mean, I think you could probably make arguments that manufacturing any of these products are going to have a negative environmental impact on one side or the other, whether you’re using fluorescents or LEDs, right?

So, I think we have to be cognizant that there’s an environmental impact either way. There is still amounts of mercury used in fluorescents, not just in HO T5 fluorescent lamps for grow lighting, but in any compact fluorescent lamps, like any of the little CFL bulbs that you use in home fixtures.

So you do need to be cognizant of that and you do need to be aware that you need to take care when you’re disposing those that they need to go to a special… Based on where you live, how they collect those sort of materials.

So that’s another valid reason, though, that you might want to make the shift, but I’m not going to tell you that manufacturing LEDs globally-

Margaret: Is a green industry [laughter.]

Leslie: …doesn’t also contribute to environmental problems. So I think you can get into that swamp on either side, for sure.

Margaret: Well, I definitely got jumped on.

Leslie: The goal is new innovation is to hopefully have a smaller footprint environmentally with more efficient types of lighting technology. Correct.

Margaret: So if either I’m someone shopping for the first time for a seed-starting setup, or I’m replacing in a newer, newish T5 high-output hood I’m looking to retrofit or whatever—if I’m shopping and I’m thinking about LEDs, the one thing that really confuses me is a lot of the fixtures…

If I were a giant greenhouse or something, there’s a lot more choices. But some of the colors! We talked already a little bit about the wattage, which confused me but which I understand a little better now, but the colors, some of them look like… I’m thinking of my kitchen counter and the light being like blue-ish, purple-ish pinkish. And I’m thinking that’s weird. That’s going to be really weird living with that for a few months of the year.

Leslie: Right. Professionally, we call it blurple, the blue-purple lights.

Margaret: Blurple? [Laughter.]

Leslie: So color of light we refer to as the spectrum, right? So if you think back to seventh-grade science class, you remember that when you shine a beam of light through a prism it breaks it into the separate spectrums of light. You have each of the different colors of light, of the rainbow that are part of that visible spectrum, and that influence photosynthesis. And each one of those colors of light has its own wavelength, frequency, and energy.

And the two colors that are… I won’t get too technical about this, but primary to driving photosynthesis and productivity are going to be the red and blue spectrum. And you can grow plants strictly under these two colors of light. They’re more efficient at driving photosynthesis. Other colors of the spectrum get used for other functions and are absorbed at smaller quantities for photosynthesis, but it’s your red and blue light that are primarily driving that process.

So if you are growing in a controlled environment—like a greenhouse or a basement or a closet or a grow tent—and that light is not going to be influencing your living space and you want it to be as efficient as possible. You’re growing lettuce or you’re growing tomatoes, and you want productivity, you might use what’s called a dual-band LED or a multi-band LED. That means it’s not full spectrum; it’s only outputting specific color spectrums of light. If it’s red and blue, you’re going to get that pink-purple colored light.

If you’re using a full-spectrum grow lamp, full spectrum means that there is some amount of every color of light included in the output and it will have a whitish look to your eye.

If there’s more red light, it might be tinted more red, and we call that a warmer spectrum. If there’s more blue-green light, then it will be a cooler light, cooler spectrum.

And so when you’re in your living space, you probably don’t want to live with purple lights [laughter]. But if you’re growing in a grow tent or a seed-starting tent then you can use the LEDs that are dual-band or multi-band that are delivering all or mostly light in the red and blue spectrum, O.K., for efficient plant growth.

Margaret: I see.

Leslie: But if you’re growing out in a kitchen area or someplace else in your living space, it’s going to be much better for you to use a full-spectrum white light, and for seed-starting, you generally want to lean towards the cool spectrum. So, anything over about 5,000K on the Kelvin rating, and usually you’re going to find lamps that are about 6,400K. Those are going to be great for seed-starting and they’re going to give you a full-spectrum, white-colored light that’s easy to live with. That make sense?

Margaret: Yes, it does. And again, I didn’t even know that it was that the “blurple” [laughter] was most-bang-for-the-buck colors. I didn’t know that. So I didn’t know why some of them were that color and some appeared white to my eyes. That’s the problem, in the products-

Leslie: Efficiency for output. So if you’re a commercial grower or you’re growing in a controlled environment, then using a tighter spectrum of light range can be beneficial for you potentially. It can be more efficient for you, right? But in a home environment, it’s better to look for a full-spectrum light that has a wider mix of colors so that it looks more natural to your eye.

Margaret: O.K. So when we go shopping, I used to know the places to look, and I used to know some of the brand names. And now it’s like brave new world. I don’t recognize any of them. And so my inclination is to, for instance, Johnny’s Selected Seeds–they used to sell like a sort of small basic rig, the Jumpstart rig with the T5 high output on it and it was inexpensive and it was good for two trays underneath it kind of a thing. And they’ve switched as of this year, they are going to be carrying these strips of these LED strips from SunBlaster another company [above, detail of a SunBlaster LED strip].

But it’s like that’s one place and one company. And then when I look on the big greenhouse supply places, the lighting-supply places, it’s like all of these brand names and stuff and I wasn’t sure.

Are there any places, any sources you want to mention or companies that you know-

Leslie: I try to be-

Margaret: Agnostic? [Laughter.]

Leslie: …cautious about sort of endorsing any particular brand, right? I don’t have any affiliations there. And I will tell you that there are well-made, quality grow lamps, and there are not-well-made quality grow lamps. And with the boom in LED technology, you have a lot of lighting gear, say, coming out of China and other places that may or may not be as higher quality.

I try to buy all of my grow lights from either U.S. manufacturers or U.S. vendors that are reputable manufacturers. So, there are some good-quality options in LED plug-and-plays and retrofits. Gardener’s Supply Company has some good options. SunBlaster is a good brand.

Gardener’s Supply lights are good, and there’s a number of others, but I will tell you, I do not buy grow lamps on Amazon [laughter]. I have had far too many problems with low quality product breakage; customer service is difficult. I really prefer to go direct to U.S. retailers, garden centers, hydroponic stores.

I will throw out a company up in Wisconsin that I found years ago that I’ve bought extensively from called Garden Supply Guys. And they have a great, easy-to-use online website, excellent customer service, good-quality products. And they have lots of different lights, lots of different LEDs, all the retrofits. So I’ll just throw that out there to them, too. And Gardener’s Supply Company. Check out Garden Supply Guys for all sorts of indoor growing gear and light. And I think that’s a good place to start.

Margaret: That’s great. Because as I said, I just followed the clue from some reputable company that sells seeds and has a lot of customers to what they choose to try to get a hint and I ended up seeing the SunBlaster name. So that’s good. That’s great.

So: How many hours do I run them [laughter]?

Leslie: O.K. So my answer is going to be, it depends because how long you wait will depend always on the lamps that you’re using—the ambient light in your space and the lamp you’re using. If you’re using a little 12-watt LED over seeds, number one, that’s never going to generate enough light, but you’d have to run it constantly. If you’re using a 250-watt CFL bulb, you could probably run it for 13 hours and be good.

A very general recommendation: If you are using, say, four 45-watt fluorescent T5 or two or three, 40- to 50-watt LED bars, you’re generally going to run those lights over seedlings that are germinating anywhere between 14 hours up to 24 hours. And I’ll talk about 24-hour lighting strategy really quick. 14 to 16 hours is a very general ballpark, good recommendation for how long to light your seedlings.

If you are growing in an otherwise dark basement, a grow tent or any other space that really doesn’t have much ambient light, you probably want to lean towards the 16 hours to potentially 18-hour length of running your lights. If you are starting those seeds in a space where there’s lots of ambient bright light in the room, you’re probably fine around 14 to 16 hours. It really is going to depend on the other environmental conditions in your space. Does that make sense?

Margaret: Yeah, yeah.

Leslie: Yeah. So 24-hour lighting strategy is something that you can do—not for tomatoes, because tomatoes reach their compensation point of being able to use that light at about 18 hours or so, 18 to 20 hours. You don’t really want to light them longer than that. But a lot of other seedlings, you can actually light 24 hours a day for the first two or three weeks of development. And that really gives them a big boost of light input and gives them a good jumpstart.

And then you can back down to that 14 to 16 hours as they continue to develop. So especially if you’re growing in an area that’s dark otherwise, or a grow tent, or a basement or something like that. So you can employ 24-hour lighting strategies on your young seedlings up to about that two- or three-week point to give them a real quick jumpstart.

Margaret: Well, Leslie Halleck, I also see… I don’t know how you have time for everything you do. You had another book come out since we spoke last and you have one called I think it’s called “Tiny Plants” coming soon. Is that right? In March?

Leslie: Yeah. “Plant Parenting” came out last year and that’s all about propagation, so if you want to dive into propagation and then I have my newest book. Yes. “Tiny Plants: Discover the Joys of Growing and Collecting Itty Bitty House Plants” [affiliate links] is coming out March 23. It’s up for pre-order now. But if you love indoor growing and you love houseplants, but are looking for something completely different, or maybe don’t have a lot of room to grow in, or you are really into terrariums or a little biophilic design, “Tiny Plants” is going to be a fun dive for you.

Margaret: Well, I’m looking forward to reading it and talking to you about it. So thank you for making time today. I know you’re super-hectic, but I so appreciate it. You’re helping me to shop smarter and to find my way through this next step in my lighting experience smarter. So thank you.

Leslie: My pleasure. If you have any follow-up questions, drop me a line.

Margaret: I will holler. Thank you.

more about light for seed-starting

• Why seedlings can get spindly and stretched-out
• How Joe Lamp’l upped his seed-starting game

enter to win ‘gardening under lights’

I’LL BUY a copy of Leslie Halleck’s “Gardening Under Lights” [affiliate link] for one lucky reader. All you have to do to enter is answer this question in the comment box at the bottom of the page:

What grow lights do you currently use for your seed-starting?

No answer, or feeling shy? Just say something like “count me in” and I will, but an answer is even better. I’ll pick a random winner after entries close at midnight Tuesday, January 26, 2021. Good luck to all.

(Disclosure: As an Amazon Associate I earn from qualifying purchases.)

prefer the podcast version of the show?

MY WEEKLY public-radio show, rated a “top-5 garden podcast” by “The Guardian” newspaper in the UK, began its 11th year in March 2020. In 2016, the show won three silver medals for excellence from the Garden Writers Association. It’s produced at Robin Hood Radio, the smallest NPR station in the nation. Listen locally in the Hudson Valley (NY)-Berkshires (MA)-Litchfield Hills (CT) Mondays at 8:30 AM Eastern, rerun at 8:30 Saturdays. Or play the January 18, 2021 show using the player near the top of this transcript. You can subscribe to all future editions on iTunes/Apple Podcasts or Spotify or Stitcher (and browse my archive of podcasts here).